Free piston pump

ABSTRACT

A thermally driven pump for pumping fluids or driving a load includes a free piston oscillating in a cylinder. A coasting region for the piston is provided by means of a bypass having displaced ports in the cylinder walls. The bypass includes a thermal regenerator. A fluid to be pumped is introduced into the cylinder through a check valve, biased to be open in response to the pressure in the cylinder being less than the pressure of the source of fluid. Closing of the check valve is delayed until the piston is wholly within the coasting region on its rebound stroke toward the coasting region. Efficiency of the device can be improved when driving an oscillating load, such as a free piston linear alternator, by providing a thermal lag cooler between the pump outlet and the oscillating load. The cooling means and oscillating load can be combined as a bellows. The temperature differential required for operating the pump can be provided by hot gas and cold gas being pumped.

Schuman Oct. 23, 1973 FREE PISTON PUMP [76] Inventor: Mark Schuman, 101G St., S.W., Wash., DC.

[22] Filed: June 20,1972

Appl. No.: 264,483

Related U.S. Application Data Continuation-in-part of Ser. No. 205,651,Dec. 7,

Primary ExaminerWilliam L. Freeh Assistant Examiner-G. P. LaPointeAttorney-Allan M. Lowe et al.

[5 7] ABSTRACT A thermally driven pump for pumping fluids or driving aload includes a free piston oscillating in a cylinder. A coasting regionfor the piston is provided by means 1971. of a bypass having displacedports in the cylinder I walls. The bypass includes a thermalregenerator. A [52] U.S. Cl 417/207, 60/24, 4l7/375 fluid to be pumpedis introduced into the cylinder [5i] Int. Cl. F04b 19/24, F03g 7/06through a check valve, biased to be open in response [58] Field ofSearch 60/24; 417/207, 375 to the pressure in the cylinder being lessthan the pressure of the source of fluid. Closing of the check valve[56] References Cited is delayed until the piston is wholly within thecoasting UNITED STATES PATENTS region on its rebound stroke toward thecoasting re- 3 563 028 2,1971 common at 31 60/24 gion. Efficiency of thedevice can be improved when 3552120 H1971 Beale 60,24 driving anoscillating load, such as a free piston linear 3:525:25 8/1970 Conrad60/24 alternator, by Providing a thermal g Cooler between 3,484,61612/1969 Baomgardner et al. 60/24 the P p Outlet and the Oscillatingload-The cooling 3,559,398 2/1971 Meijer et al. 60/24 means andoscillating load can be combined as a bel- 3,583,155 6/1971 Schuman60/24 lows. The temperature differential required for operat- 3,604,8 2l9/1971 Martini 60/24 ing the pump can be provided by hot gas and coldgas 3.597.766 8/1971 BUCk..... 60/24 being pumped 3,608,311 9/1971Roesel 60/24 I 24 Claims, 3 Drawing Figures STORAGE TANK 451 52 1:

THERMAL LAG COOLER LOAD "L 'lOb 'l'il:

l4ll I 1 1 s 1 A BB 655 |5b b 6gb J1 Q PAIENIEDoma am 3.76? 325 rTORAeE1 ANK 61a Gig l Hal Y VT? 496 v TIQ 'IOa

mew

LAG COOLER LOAD 7 80 g ll 8t LOAD H f 18 COOLER. '1

FREE PISTON PUMP RELATIONSHIP TO COPENDING APPLICATION The presentapplication is an improvement and continuation-in-part of my copendingapplication entitled Free Piston Apparatus, filed Dec. 7, 1971, Ser. No.205,651.

BACKGROUND OF INVENTION In the copending application, there is discloseda thermally powered compressible fluid pump wherein a free pistonoscillates in a cylinder between ends of the cylinder. A coasting regionin the cylinder is provided by a cylinder bypass having a pair ofdisplaced ports in the cylinder between the cylinder ends. The bypassmeans is provided for obtaining a temperature difference forcompressible fluid at opposite ends of the cylinder. The piston shuntsthe fluid in alternate directions through the bypass and regeneratorbetween the heating and cooling means, or sources of hot and cold fluid,at opposite ends of the cylinder to produce alternating pressure forpumping fluid or driving a load. Compressible fluid to be pumped may beintroduced into the cylinder through a check valve biased to be open inresponse to pressure within the cylinder being approximately equal to orless than the pressure of a source of the compressible fluid. Thereby,the compressible fluid at the inlet generally is admitted into thecylinder while the piston is moving in a direction tending to cool thegas and decrease pressure within the cylinder. A port is provided toexhaust fluid through an outlet check valve in response to the cylinderpressure being higher than the pressure at an outlet of the check valve.

1 While the piston is within the coasting region the pressures atopposite faces of the piston are substantially equal at any given time,because of the bypass around the piston. Thereby, pressure variationsduring coasting do not tend to stall the piston. However, during reboundof the piston at either end of the cylinder, the bypass is blocked atone end by the piston wall, so that pressure is generally not equal atopposite faces of the piston and may tend to stall the piston if pumpingwork is attempted during a rebound portion of the cycle. It is thusdesirable in general to delay the closing of any inlet check valve whichopens during piston rebound, so that compression by the piston of thegas drawn into the cylinder through the inlet does not begin until thepiston has reached the, coasting region. If fluid is being pumped duringrebound, the piston, at this time, functions to a certain extent as aworking piston rather than solely as areversing piston. In piston pumpsof the type disclosed in my copending application, it is desirable forthe piston, while it is in the rebound region, to function exclusivelyas a reversing piston, and not as a working piston because of thepossibility of the piston being stalled or decreasedin frequency oramplitude of oscillation by the gas it is compressing. Since the pistonis preferably driven while in the rebound region by a thermal lagheating chamber which appears to be a relatively low efficiency device,energy cannot be efficiently transferred to a gas being compressed, andthe pumping efficiency is low during rebound. However, while the pistonis moving through the coastin'g region, it functions in a very efficientmanner, as a Stirling type displacer piston to convert thermal energytolpneumatic energy. Thereby, it is desirable to pump the compressiblefluid while the piston is moving through the coasting region but it isgenerally not desirable to admit and then compress fluid, while thepiston is in the rebound region.

BRIEF DESCRIPTION OF THE INVENTION In accordance with the presentinvention, the aforementioned pump of my copending application ismodified so that the volume above an upper face of the piston is ventedas the piston moves toward an entry location in the cylinder for thefluid. Venting occurs until all of the piston is within the coastingregion so that substantially no fluid drawn into the cylinder throughthe inlet is compressed by the upper face of the piston until the pistonenters the coasting region. Thereby, stalling of the piston due to backpressure building up against the piston upper face is precluded. Inaddition, efficiency of the device is maintained at a relatively highlevel since the fluid is compressed only while the piston functions as adisplacer piston as it moves through the coasting region and not whilethe piston functions as a reversing piston when it is in the reboundregion.

In a preferred embodiment, the coasting region is vented through thesimple mechanism of delaying the closing time of a check valve providedin a conduit connecting the fluid entry location into the cylinder withthe source of fluid. The delay time is such that the check valve, whenis normally open in response to the pressure from the source exceedingthe pressure in the cylinder, does not close until the piston is whollywithin the coasting region during its movememt back toward the entrylocation.

A further feature of the invention resides in providing a pair of pistoncylinder pumps operated in synchronism so that the two pistons approacha central thermal lag heating chamber atthe same time. The thermal lagheating chamber can function as the sole heating means.

According to an additional feature, efficiency of a regenerative freepiston device connected to an oscillating load is enhanced by providinga thermal lag cooler in circuit with the load. If the load is a bellows,the bellows also functions as'the thermal lag cooler. For other types ofoscillating loads (e.g., a pressure driven alternator) a twin path,check valves, and a cooling chamber are used between the pump and theload whereby gas flowing from the pump to the load is not cooled but gasreturning to the pump from the load is cooled. Thereby, pressure offluid is reduced by cooling only after it has done its work on the loadand is flowing back into the pump for later heating. The thermal lagcooler does this to a lesser extent.

It is, accordingly, an object of the present invention to provide a newand improved pump utilizing compressible fluid.

Another object of the present invention is to provide a new and improvedcompressible fluid pump employing at least one free piston which has avery low probability of being stalled.

A further object of the present invention is to provide.

a new and improved thermally driven free piston regenerative cycle pumphaving a relatively An efficiency. An additional object of the inventionis to provide a thermally driven free piston pump wherein a cylindercontaining the piston is vented in a facile manner as the piston isrebounding toward the coasting region within a cylinder bypass region.

Another object of the invention is to provide a thermally driven pistonapparatus wherein an oscillating load also functions as a cooler toenhance efficiency.

An additional object is to provide an efficient thermally driven pumpfor driving an oscillatory load wherein means are provided for coolingfluid flowing from the load to the pump.

Yet a further object of the present invention is to provide a thermallydriven free piston pump wherein fluid drawn into the pump is compressedonly while the piston is acting as a displacer piston.

Still another object of the present invention is to provide a thermallypowered free piston pump having means for delaying the closing of aninlet check valve to substantially avoid compression work by the pistonduring rebound.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of several specific embodiments thereof,especially when taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of apreferred embodiment of the present invention;

FIG. 2 is a schematic diagram of a segment of the FIG. 1 embodiment inaccordance with a modification wherein check valves, twin conduits, anda cooler are employed to cool fluid returning to the pump from anoscillatory load; and

FIG. 3 is a schematic diagram of a segment of the FIG. 1 embodiment, inaccordance with a further modification wherein an oscillating bellowscomprises the oscillating load and cooling means.

In the drawing, a free piston pump is illustrated as containing a pairof cylinders and a pair of synchronized free pistons. It is to beunderstood, however, that the principles of the invention are applicableto a pump having any number of piston cylinder combinations, i.e., thepump may include only one piston cylinder combination or it may includemore than two piston cylinder combinations interconnected in the samemanner as the pair of piston cylinder combinations illustrated.

DETAILED DESCRIPTION OF THE DRAWING Reference is now made to FIG. 1 ofthe drawing wherein there is illustrated a gas pump including a pair ofsealed cylinders 11a and 11b through which free pistons 12a and 12b arerespectively oscillated in synchro nism with each other so that bothpistons approach and recede from adjacent faces of cylinders 11a and 11bsimultaneously. (In the description, corresponding parts associated withthe upper and lower cylinders 11a and 11b are provided with the suffixletters a and b, respectively. If no mention is made in the descriptionof the drawing of the suffix letters a and b and the part is included inor associated with the cylinders 11a and 11b, it is to be understoodthat the elements of both cylinders are being considered.) The oppositefaces 13 and 14 of piston 12 form end walls for first and secondchambers 15 and 16 of cylinder 11.

The walls of cylinder 11 and piston 12 are so close to each other thatthe piston provides a reasonably good seal between the opposite facesthereof. Therefore, while piston 12 is near the end walls 17 and 18 ofchambers 15 and 16, respectively, there is a substantial differencebetween gas pressures at opposite faces of the piston. In a centralregion of cylinder 11, the pressure difference between faces 13 and 14of piston 12 is virtually zero due to a bypass formed by conduits 21 and22, between which is connected regenerator 23. Lines or conduits 21 and22 are connected to the side wall of cylinder 11 at disparate pointsalong the cylinder by ports 24 and 25. The length of piston 12 isdetermined in such a manner that both conduits 21 and 22 communicatewith chambers 15 and 16 while piston 12 is coasting in the centralregion between the ports. The positions of ports 24 and 25 and thelength of piston 12 are such that the piston covers and blocks conduit21 while the volume in chamber 15 is a minimum, and for a certain timeinterval on either side of the minimum volume; and the piston blocksconduit 22 while the volume in chamber 16 is a minimum and for a certaintime interval on either side of the minimum volume. Chambers 15, 16 andthermal lag heating chamber 31 form gaseous springs at each end ofcylinder 11 so that compressible fluid in chambers 15 and 16 when thechamber volumes are minimized result in rebound of piston 12 from endfaces 17 and 18 of cylinder 11.

Synchronized oscillation of pistons 12a and 12b is sustained by athermal device comprising thermal lag heating chamber 31 having acentrally located heating core 32. Surrounding heating core 32 areelongated fluid passageways 33 within a housing 34. Heated passageways33 are in fluid flow relationship with the end faces 18a and 18b ofchambers 16a and 16b by virtue of conduits 35 being connected to acommon T element 36 having one leg connected to conduit 37 whichprovides a fluid flow path from the interior of thermal lag heatingchamber 31 to conduits 35. As described in my copending application,Oscillating Piston Apparatus, Ser. No. 227,514, filed Feb. 18, 1972, thethermal lag heating chamber is responsive to a periodic surge ofcompressible fluid resulting from piston face 14 coming into proximitywith cylinder end wall 18. The compressible fluid is forced throughconduits 3S and 37 to the interior of thermal lag heating chamber 31.The fluid resides in the thermal lag heating chamber 31, is heatedtherein, and, while still being heated, expands out of the thermal lagheating chamber as piston 12 is rebounding near end wall 18.

Sources of a cold and a hot fluid, e.g., air, to be pumped by theapparatus of the present invention feed chambers 15 and 16 throughconduits 41 and 42 and valves 43 and 44. The cold and hot fluids passingthrough valves 43 and 44 are introduced into chambers 15 and 16,respectively, via check valves 45 and 46, conduits 47 and 48, and ports49 and 50. Ports 49 and 50 are respectively aligned with ports 24 and 25leading to conduits 21 and 22 of the bypass. Thereby, the entirecoasting portion of the cycle is available for intake of fluid. Further,there is less leakage around the piston during rebound near end faces 17and 18 than if ports 49 and 50 were closer to these end faces. Valves 45and 46 are polarized so that they are normally open in response to thepressure of the gas emerging from valves 43 and 44 exceeding thepressure within chambers 15 and 16; conversely, valves 45 and 46 arenormally closed in response to the pressure within chambers 15 and 16exceeding the pressure of gas at the inlets of check valves 45 and 46.

Valve 45 is provided with delay means to prevent closure thereof as soonas the pressure in chamber exceeds the pressure of the gas at the inletof valve 45.

The closure of valve 45 is delayed for a time equal to the'travel timeof piston 12 from adjacent the end wall 18 of cylinder 11 until thebypass has been completely established as the piston is moving towardinlet port 49. By delaying closure of valve 45 until the bypass regionhas been established, stalling of piston 12 is virtually precludedbecause face 13 of piston 12 sees a substantially constant pressurethroughout piston rebound near cylinder end wall 18. Since chamber 15 isvented in response to valve 45 being open no substantial net pumping ofgas occurs, and there is no substantial positive net intake of gas intochamber 15, while the rebound chamber below face 14 is established. Gasis pumped when piston 12 reaches the coasting region by the displacementof gas from chamber 15 to chamber 16 by piston 12. Piston 12 would notfunction efficiently as a pump while the bypass is closed, when chambers15 or 16 are rebound chambers, because of the relatively inefficientnature of thermal lag heating chamber 31.

Two separate types of delay means are illustrated for check valves 45aand 45b to provide an increased number of examples of possible delaymeans which may be utilized. It is to be understood, however, that in anactual device, the two delay means would be preferably the same. Thedelay means of check valve 45a comprises a spring 52a having oppositeends connected to wafer 53a and stem 54a of adjustable plug 55a. Theamount of delay in the closure of wafer 53a is determined by theposition of stem 54a in the housing of valve 45a. The delay for valve45b is provided by appropriately positioning stop 56b for wafer 53brelative to the wafer seat against wall 60b of the housing of valve 45b.

The selection of whether to employ spring means or a selected backstop-seat distance as the delay device for check valve 45 involves anumber of considerations. The spring 52a of valve 45a, may not be assensitive to orientation of the device in a gravitational field as isfree floating wafer 53b, and may then provide a more positive andpredictable delay time which can be adjusted at will by controlling theposition of stem 54a. The spring, however, has the disadvantage of beingmore complex than the wafer, back stop valve 45b. A further, and perhapsmore important, disadvantage of the spring is that during each cycle ofpiston operation, two and possibly three fluid pulses flow backwardthrough check valve 45a; generally only a single pulse of fluid flowsbackward through the check valve if wafer back stop check valve 45b isemployed. The extra pulse or pulses occur if the spring is employedbecause the spring biases wafer 53a to cause the wafer to open inresponse to the pressure in chamber 15 or conduit 47a not beingsufficiently greater than that of the source to overcome springcompression, which generally occurs after piston 12 has entered thecoasting region on its travel away from wall 17 and also as the pistonis rebounding toward the coasting region.

In operation, the compressible fluid in chambers 15 and 16 isalternately and cyclically cooled and heated as piston 12 coasts awayfrom and towards cylinder end face 17. Fluid in chamber 15 is cooled inresponse to cold fluid flowing into chamber 15 through port 49; andfluid flowing into chamber 15 through the bypass is cooled byregenerator 23 and optional cooling chamber 58. Fluid in chamber 16 isheated in response to hot fluid flowing through port 50; fluid flowinginto chamber 16 through the bypass is heated by regenerator 23 andoptional heating chamber 59; and fluid flowing from chamber 16 intothermal lag heating chamber 31 is heated by thermal lag passageways 33.Cold and hot chambers 58 and 59 are respectively located in conduits 21and 22 such that fluid flowing in conduits 21 and 22 must pass throughchambers 58 and 59. Pumping power and performance can be controlled byvarying the amount of cooling and heating of fluid by chambers 58 and 59by providing variable shunt paths 61 and 62 around the cold and hotchambers. The flow of fluid through chambers 58 and 59 is controlled bythree-way variable ratio valves 63 and 64 which respectively connectconduit 21 with conduit 61 and conduit 22 with conduit 62. Conduits 61and 62 shunt fluid around chambers 58 and 59 in conduits 21 and 22 sothat, in conjunction with valves 63 and 64, any fraction (from zero toone) of the fluid flowing in conduits 21 and 22 can be diverted aroundchambers 58 and 59 without increasing the flow impedance of the bypass.

To enable fluid compressed by piston 12 within cylinder 11 to beevacuated from the cylinder, the cylinder includes outlet ports 65 and66 respectively aligned with inlet ports 49 and 50. Ports 65 and 66 arecircumferentially displaced from ports 49 and 50, as well as ports 24and 25, to emphasize the fact that the location of any port can beindependently varied along or around the cylinder axis. Port locationsother than those shown in FIG. 1 are feasible.

Ports 65 and 66 are respectively connected to check valves 67 and 68,having outlets connected to storage tanks 69 and 70. Check valves 67 and68 are arranged so that the wafers 71 and 72 thereof are open only inresponse to the pressure within chambers 15 and 16 exceeding thepressures within tanks 69 and 70. It is to be understood that, ifdesirable, tanks 69a and 69b can be a single tank or other load drivenin parallel; and that tanks 70a and 70b can also be combined. Suchcombining of like loads helps balance the multi-piston pump about itsaxis of symmetry and generally improves piston synchronization. Symmetryof design about the axis of symmetry generally contributes to pistonsynchronization. Instead of, or in addition to, supplying the gas fedthrough check valve 68 to storage tank 70 or other load, this gas can beheated by solar radiation and returned to the hot fluid inlet 42, inorder to provide heat for operating the pump. 1n such a configuration,the two conduits carrying hot gases from valves 68a and 68b arepreferably combined in a T element, heated and then supplied to the hotinlets 42a and 42b after passing through a further T element. Thisexternal heating loop, which may contain an additional load to be drivenby hot gas, can also supply heat to thermal lag heating chamber 31 bypassing the loop through a heat exchanger formed on chamber 31. The coldgases fed through check valves 67a and 67b can also be combined in a Telement, cooled by an ambient air, water, radiative, or other coolingmeans, and recirculated back to inlets 41a and 41b after passing througha further T and optional load. Thus solar heating and ambient coolingcould provide the sole thermal energy for operating the pump. Solarenergy can thus be converted by this device into pneumatic energy andthence into electrical or other form of energy.

To describe the operation of the device, initially assume that piston 12is oscillating in cylinder 11, with oscillation being started by apneumatic or other impulse as described in Ser. No. 205,651. Also,assume that piston face 13 is adjacent and has just begun moving awayfrom cylinder end wall 17. At this time, piston 12 blocks bypass port24, inlet port 49, outlet port 65, and port 76 to substantiallyeliminate fluid flow between the cylinder and these ports duringrebound, and to avoid pumping work by the piston which might tend toslow or stall the piston. Thereby, the pressure in chamber is now in amaximum range while the pressure in chamber 16 is less than in chamber15.

Next, assume that the cycle has progressed so that piston 12 has movedaway from end wall 17 and has just entered the coasting region, wherebyports 24, 49, 65 and 76 are open. Thereby, the bypass is established andhot gas flows into the bypass from chamber 16 and cooled gas flows fromthe bypass into cold chamber 15. This cooling of gas in the bypassdecreases the pressure in chambers 15 and 16. The pressures on theopposite faces 13 and 14 of piston 12 are substantially equalizedbecause of the low impedance path of the bypass. When piston 12initially moves into the coasting region, valve 45 remains closed orcloses after a short delay because the pressure in chamber 15 exceedsthat of the source connected to conduit 41. As the piston moves fartherthrough the coasting region, the decreasing pressure within chamber 15becomes almost equal to the pressure of the source connected to theconduit 41, whereby valve 45a opens by virtue of its spring compressionand fluid flows from chamber 15 back to the source of cold fluid. Aspressure in chambers 15 and 16 become lower than the cold and hot fluidsources, fluid from the cold and hot sources is drawn into chambers 15and 16 through valves 45 and 46. Cold fluid continues to flow throughvalve 45 from the source connected to conduit 41 as piston 12 continuesto move away from wall 17 and out of the coasting region into therebound region near cylinder end wall 18.

When piston 12 is in the rebound chamber, the bypass is cut off due tothe sealing action of piston 12 against port 25. When the reboundchamber volume is minimized, and chamber 15 pressure approximately atits minimum, piston 12 begins to move away from cylinder end wall 18, byvirtue of a pneumatic spring between face 14 and end wall 18, as well asin response to the pressure supplied to this piston face 14 by thermallag heating chamber 31. As piston 12 moves upwardly through the reboundregion, the pressure in chamber 15 increases, tending to close valve 45.The valve, however, does not close because of its delayed closureaction, as described supra. Thereby, a relatively constant and low backpressure acts against piston face 13, to prevent stalling of piston 12.Valve 45 continues to remain open until piston 12 has moved into thecoasting region of cylinder 11, at which time the backflow of gasthrough check valve 45 is sufficient to close valve 45. Because valve 45is open during this entire rebound portion of the cycle, there is asubstantially zero net flow of gas into chamber 15 during this rebound,and substantially zero pumping work by the piston. Alternatively, valve45 could be solenoid operated by a sensor responsive to piston positionso as to be closed or open during the entire time of this rebound; inthe former case the valve would not have substantial delaycharacteristics. As piston 12 moves through the coasting region towardcylinder end wall 17, the pressure in chamber 15 increases as a resultof the heating of gas flowing from chamber 15 to chamber 16 via thebypass. The increasing pressure in chamber 15 becomes suffciently greatto open valves 67 and/or 68, whereby fluid is fed to loads 69 and/or 70.Valves 67 and 68 remain open while piston 12 is in the coasting regionapproaching end wall 17.

In response to piston face 13 passing a plane defined by ports 24, 49and 65 in the travel of the piston toward end wall 17, port 65 becomesblocked, whereby fluid no longer flows to load 69. Cylinder 12, at thistime, enters a second rebound chamber formed between piston end face 13and cylinder end wall 17, while ports 24, 49 and 65 are blocked by thepiston side walls. The pressure in the second rebound chamber increasesuntil a pneumatic spring between piston face 13 and end wall 17 issufficiently great to reverse the motion of the piston, whereby thepiston begins to move back towards cylinder end wall 18.

Valve 46, connecting the hot source to chamber 16, generally opens andcloses in synchronism with opening and closing of valve 45 while piston12 is in the coasting region. This is because the pressures in chamber15 and 16 are substantially the same while piston 12 is in the coastingregion. While piston 12 is in the rebound chamber defined by the volumebetween piston face 14 and cylinder end wall 18, flow through valve 46is restricted by piston 12 blocking port 50. Valve 46 is generallyclosed while piston 12 is in the rebound chamber defined between pistonface 13 and cylinder end wall 17 because the chamber 16 pressure isgenerally greater at this time than the pressure of the source of hotfluid. The small drop in chamber 16 pressure during this rebound portionof the cycle is generally insufficient to cause valve 46 to open. If,however, the outlet of valve 67 or valve 68 or conduit 77 is connectedto a low impedance load, such as the atmosphere in an extreme case, thepressure in chambers 15 and 16 remains relatively constant as piston 12is moving through the coasting region toward end wall 17. Thereby, inresponse to piston 12 entering the rebound region between piston endface 13 and cylinder end wall 17, the pressure in chamber 16 drops toslightly less than that of the hot fluid source connected to conduit 42,whereby there is flow through valve 46 at this time. Intake andcompression of gas in chamber 16 during this rebound would tend to slowor stall piston 12, which is supposed to function as a displacer pistonand not a working piston. Under these conditions, therefore, it may bedesirable to provide a delay for valve 46,

using means similar to those described for valve 45. The delay time canbe adjusted in the same manner as the delay time for valve 45, i.e., toequal to the time while piston 12 is rebounding from near cylinder endwall 17 toward the coasting region.

In certain instances, it is desirable to utilize cylinder 11 to drive anoscillatory load 75, such as an alternator of the type disclosed in mycopending application Ser. No. 205,651. In such an instance, no checkvalves need be provided between the cylinder outlet ports and the loadand load is connected to be driven by fluid pumped through ports 76a and76b to conduits 77a and 77b which are connected to T element 78 that inturn feeds gas through conduit 79 to the load. In response to theoscillatory pressure variations in conduit 79, the load 75 is cyclicallydriven to perform useful work. An

alternate or second load can be connected to port 66 to be driven byhot, rather than cool, gas.

To increase the efficiency of the device and reduce losses due toheating of conductors and magnetic material of the alternator, optionalthermal lag cooler 80 is connected in line 79 to cool gas flowingbetween load 75 and cylinder chamber 15. Thermal lag cooler 80 isprovided with relatively wide fluid passageways, as described in mypreviously referenced copending applications. Cooling of the passagewayscan be performed by substituting a cooling element for heating element32 of chamber 31 and by providing an inlet and outlet on opposite sidesof the chamber. Alternatively, cooling fins which are in heat exchangerelationship with ambient air can be provided for cooling. Because thethermal lag cooler includes relatively wide passageways, much of thecooling occurs after the gas has done its work on the load at relativelyhigh pressure and is returning from the load to chamber at decreasingpressure for later heating and pressure increase within the pump. Therelatively great passageway width also reduces fluid drag of the coolingmeans.

In FIG. 2 there is illustrated an alternative arrangement for coolinggas returning from the oscillatory load to the cold end of the cylinder,wherein check valve 81, biased to pass fluid from cooling chamber 180,which can be of either'the conventional or thermal lag type, to Telement 78 in response to the pressure of fluid in the pump droppingbelow that of the load 75, is connected between the cooler and Telement. A further check valve 82,'polarized to pass fluid from Telement 78 to load 75 in response to pump pressure exceeding loadpressure, is connected between the T element and load in parallel with'cooler 180 and check valve 81.

In accordance with a further aspect of the invention the oscillatingload is bellows 85, FIG. 3, connected directly to T element 78 viaconduit 79. Bellows 85 is inherently a thermal lag cooling device thatcools fluid within the bellows folds while supplying the fluid back tocylinder chamber 15 for subsequent heating and pressure increase.

While there has been described and illustrated several specificembodiments of the invention, it will be clear that variations in thedetails of the embodiments specifically illustrated and described may bemade without departing from the true spirit and scope of the inventionas defined in the appended claims. For example, it is not necessary tofeed gas into either end of cylinder 11. Compressible fluid can beintroduced into only one end of the cylinder or it can flow to and fromthe ends of the cylinder solely from the bypass. Also,

the fluid flowing through conduits 41 and 42 need not necessarily bederived from cold and hot sources. However, in order to operate thepump, a means must be provided to obtain a difference in temperaturebetween fluid in opposite ends of the cylinder. Thus there must be atleast one means for cooling and at least one means for heating. Thecooling can be a source of cool fluid, a cooling chamber in the bypass,or a cooling means between the pump and an oscillatory load as describedsupra, or any combination of these cooling means. Correspondingly, themeans for heating can be a source of hot fluid, a heater in the bypass,a thermal lag heating chamber, a heating means associated with a load orbetween the pump and a load, or any combination of these heating means.

If there is a fluid inlet to the pump there must, of course, be a fluidoutlet. The outlet can be at the cold end of the cylinder, the hot end,or both, irrespective of whether the inlet is at the cold end, the hotend, or at both ends of the cylinder or coasting region. An oscillatoryload, such as a bellows, can be driven whether or not inlet and outletports and associated check valves are provided, since the net flow to astrictly oscillatory load is zero. However, a long bypass containing aregenerator, for coasting of the piston and modified regenerativethermodynamic cycle output, is generally desirable in any embodiment ofthis invention illustrated or discussed above in order to obtainrelatively high energy conversion efficiency and stall-free operation.

lt should be understood that other types of delayed closing valve, suchas a ball type check valve, can be used instead of the wafer checkvalve. If spring biasing is not utilized, the distance of travel of theball or wafer from its backstop to its seat, as well as the mass of thewafer, must be great enough for adequate delay.

I claim:

1. A thermally driven pump utilizing compressible fluid, comprising acylinder, a free piston in the cylinder, means for obtaining atemperature differential between fluid in opposite ends of the cylinder,means for sustaining oscillation of the piston in the cylinder, a fluidbypass having displaced ports between ends of the cylinder, whereby acoasting region for the piston is formed between the ports, aregenerator in the bypass, a rebound chamber near each end of thecoasting region, inlet means for feeding fluid from a source of fluidinto a portion of the cylinder, said inlet means including valve meansfor connecting the cylinder in fluid flow relationship with the sourceof fluid only while the pressure within the portion is less than thepressure of the source of fluid, and means for venting the coastingregion fora selected time interval while the piston is in a reboundchamber and moving toward the coasting region.

2. The pump of claim 1 wherein the selected time interval issubstantially the entire time while the piston is in the rebound chamberand moving toward the coasting region.

3.'The pump of claim 1 wherein the means for sustaining oscillationincludes a thermal lag chamber.

4. The pump of claim 1 wherein the means for venting includes a checkvalve.

5. The pump of claim 1 wherein the means for venting includes a springbiased check valve.

6. The pump of claim 1 wherein the means for ventingcomprises a checkvalve including a'wafer and a back stop.

7. The pump of claim 1 wherein the means for ob taining a temperaturedifferential includes means for feeding cool fluid into the cylindernear one end of the cylinder, and means for feeding hot fluid into thecylinder near the other end of the cylinder.

8. The pump of claim 7 wherein the means for sustaining oscillationincludes a thermal lag chamber.

9. The pump of claim 1 further including outlet means for feeding fluidto a load.

10. The pump of claim 1 further including means for supplying fluid to aload and means for cooling fluid flowing from the load to the pump.

11. The pump of claim 10 wherein the means for cooling includes athermal lag cooling chamber.

12. The pump of claim 11 wherein the load and the thermal lag coolingchamber are combined as a bellows.

13. A thermally powered pump utilizing compressible fluid comprising aplurality of cylinders, a free piston in each of the cylinders, meansfor sustaining synchronized oscillation of the ,pistons in thecylinders, a fluid bypass for each cylinder, each bypass havingdisplaced ports for allowing fluid to bypass a portion of its respectivecylinder between ends of the cylinder, whereby a coasting region foreach piston is formed between the displaced ports of the respectivecylinders, a rebound chamber for each piston near each end of thecylinder, inlet means for normally providing a fluid flow path from afluid source into a portion of each cylinder only while the pressurewithin the portion is less than the pressure of the source of fluid, andmeans for venting the coasting region of each cylinder during a selectedtime interval of the rebound portion of the oscillation cycle.

14. The pump of claim 13 wherein the means for sustaining includes athermal lag heating chamber in fluid flow relation with a reboundchamber of each cylinder.

15. The pump of claim 13 further including means for feeding cold fluidinto each cylinder near one end of each cylinder and means for feedinghot fluid into each cylinder near the other end of each cylinder.

16. In combination, a cylinder, a piston in the cylinder, means,including heating means, for sustaining oscillation of the piston in thecylinder, a load responsive to fluid compressed by the piston in achamber adjacent one end of the cylinder, said load including a bellowsin fluid flow relation with and driven in response to fluid in saidchamber, whereby the cooling of fluid within the bellows as the fluid isreturning to the chamber assists in supplying energy to the load.

17. The combination of claim 16 further including means for feeding hotfluid into a cylinder chamber adjacent the opposite end of the cylinder.

18. The combination of claim 17 wherein the piston is a free piston andfurther including a bypass for the cylinder between the chambers wherebya coasting region is established for the free piston within the bypassregion of the cylinder, and a regenerator in said bypass.

19. The combination of claim 16 wherein the means for sustainingoscillation includes a thermal lag heating chamber.

20. An oscillating piston apparatus comprising a cylinder, a free pistonin the cylinder dividing the cylinder into first and second variablevolumes, means for sustaining oscillation of the piston in the cylinder,a bypass between the first and second volumes such that the pistoncoasts through a region of the cylinder between ends of the cylinder, aregenerator means in the bypass, means for blocking the bypass duringthe piston oscillation while at least one volume has a value in aminimum range, means for feeding cool fluid into the second volume,means for feeding hot fluid into the first volume, means for supplyingfluid from the cylinder to a load, and means for cooling fluid returningto the cylinder from the load.

21. The apparatus of claim 20 wherein the means for cooling returningfluid includes a thermal lag cooling chamber located in a conduitconnecting the cylinder and the load.

22. The apparatus of claim 20 wherein the means for supplying includesan outlet conduit and check valve for passing fluid only from thecylinder to the load, and means, including a return conduit, coolingchamber, and check valve, for passing fluid only from the load throughthe cooling chamber and check valve to the cylinder.

23. The apparatus of claim 20 wherein the load and the means for coolingreturning fluid are combined in a bellows.

24. The apparatus of claim 20 further including valve means forpreventing a substantial net flow of fluid into the variable volumecontaining the coasting region while the piston is in a rebound regionbeyond the coasting region.

1. A thermally driven pump utilizing compressible fluid, comprising acylinder, a free piston in the cylinder, means for obtaining atemperature differential between fluid in opposite ends of the cylinder,means for sustaining oscillation of the piston in the cylinder, a fluidbypass having displaced ports between ends of the cylinder, whereby acoasting region for the piston is formed between the ports, aregenerator in the bypass, a rebound chamber near each end of thecoasting region, inlet means for feeding fluid from a source of fluidinto a portion of the cylinder, said inlet means including valve meansfor connecting the cylinder in fluid flow relationship with the sourceof fluid only while the pressure within the portion is less than thepressure of the source of fluid, and means for venting the coastingregion for a selected time interval while the piston is in a reboundchamber and moving toward the coasting region.
 2. The pump of claim 1wherein the selected time interval is substantially the entire timewhile the piston is in the rebound chamber and moving toward thecoasting region.
 3. The pump of claim 1 wherein the means for sustainingoscillation includes a thermal lag chamber.
 4. The pump of claim 1wherein the means for venting includes a check valve.
 5. The pump ofclaim 1 wherein the means for venting includes a spring biased checkvalve.
 6. The pump of claim 1 wherein the means for venting comprises acheck valve including a wafer and a back stop.
 7. The pump of claim 1wherein the means for obtaining a temperature differential includesmeans for feeding cool fluid inTo the cylinder near one end of thecylinder, and means for feeding hot fluid into the cylinder near theother end of the cylinder.
 8. The pump of claim 7 wherein the means forsustaining oscillation includes a thermal lag chamber.
 9. The pump ofclaim 1 further including outlet means for feeding fluid to a load. 10.The pump of claim 1 further including means for supplying fluid to aload and means for cooling fluid flowing from the load to the pump. 11.The pump of claim 10 wherein the means for cooling includes a thermallag cooling chamber.
 12. The pump of claim 11 wherein the load and thethermal lag cooling chamber are combined as a bellows.
 13. A thermallypowered pump utilizing compressible fluid comprising a plurality ofcylinders, a free piston in each of the cylinders, means for sustainingsynchronized oscillation of the pistons in the cylinders, a fluid bypassfor each cylinder, each bypass having displaced ports for allowing fluidto bypass a portion of its respective cylinder between ends of thecylinder, whereby a coasting region for each piston is formed betweenthe displaced ports of the respective cylinders, a rebound chamber foreach piston near each end of the cylinder, inlet means for normallyproviding a fluid flow path from a fluid source into a portion of eachcylinder only while the pressure within the portion is less than thepressure of the source of fluid, and means for venting the coastingregion of each cylinder during a selected time interval of the reboundportion of the oscillation cycle.
 14. The pump of claim 13 wherein themeans for sustaining includes a thermal lag heating chamber in fluidflow relation with a rebound chamber of each cylinder.
 15. The pump ofclaim 13 further including means for feeding cold fluid into eachcylinder near one end of each cylinder and means for feeding hot fluidinto each cylinder near the other end of each cylinder.
 16. Incombination, a cylinder, a piston in the cylinder, means, includingheating means, for sustaining oscillation of the piston in the cylinder,a load responsive to fluid compressed by the piston in a chamberadjacent one end of the cylinder, said load including a bellows in fluidflow relation with and driven in response to fluid in said chamber,whereby the cooling of fluid within the bellows as the fluid isreturning to the chamber assists in supplying energy to the load. 17.The combination of claim 16 further including means for feeding hotfluid into a cylinder chamber adjacent the opposite end of the cylinder.18. The combination of claim 17 wherein the piston is a free piston andfurther including a bypass for the cylinder between the chambers wherebya coasting region is established for the free piston within the bypassregion of the cylinder, and a regenerator in said bypass.
 19. Thecombination of claim 16 wherein the means for sustaining oscillationincludes a thermal lag heating chamber.
 20. An oscillating pistonapparatus comprising a cylinder, a free piston in the cylinder dividingthe cylinder into first and second variable volumes, means forsustaining oscillation of the piston in the cylinder, a bypass betweenthe first and second volumes such that the piston coasts through aregion of the cylinder between ends of the cylinder, a regenerator meansin the bypass, means for blocking the bypass during the pistonoscillation while at least one volume has a value in a minimum range,means for feeding cool fluid into the second volume, means for feedinghot fluid into the first volume, means for supplying fluid from thecylinder to a load, and means for cooling fluid returning to thecylinder from the load.
 21. The apparatus of claim 20 wherein the meansfor cooling returning fluid includes a thermal lag cooling chamberlocated in a conduit connecting the cylinder and the load.
 22. Theapparatus of claim 20 wherein the means for supplying includes an outletconduit and check valve for passing fluid only from the cylinder to theload, aNd means, including a return conduit, cooling chamber, and checkvalve, for passing fluid only from the load through the cooling chamberand check valve to the cylinder.
 23. The apparatus of claim 20 whereinthe load and the means for cooling returning fluid are combined in abellows.
 24. The apparatus of claim 20 further including valve means forpreventing a substantial net flow of fluid into the variable volumecontaining the coasting region while the piston is in a rebound regionbeyond the coasting region.